Dynamic interbody

ABSTRACT

An intervertebral implant includes a body having an upper surface, a lower surface and a transverse fastener hole. The body includes a dynamic fixation mechanism for seating a fastener in the fastener hole and translating the fastener relative to the implant to accommodate subsidence of the implant. In one embodiment, an implant includes a transverse slot with an elongated seat for translating a seated fastener relative to the implant. In another embodiment, an implant includes a displaceable sliding member for translating a fastener relative to the implant.

FIELD OF THE INVENTION

The present invention relates generally relates to intervertebralimplants, and more specifically to interbody components with mechanismsfor dynamically transmitting loads during interbody subsidence.

BACKGROUND OF THE INVENTION

Anterior lumbar interbody fusion (ALIF) is a common technique fortreating degenerative discs from an anterior approach. The anteriorapproach allows access to the interbody space with minimal damage to theposterior musculature, while allowing full decompression of the diseaseddisc. The ALIF procedure has been used with lumbar plates and cages withscrews rigidly affixed within the construct. A number of cages includean interbody with a hollow or open area in the center which receivesbone graft material. The bone graft material fuses the adjacentvertebrae is together.

SUMMARY OF THE INVENTION

In accordance with one exemplary embodiment of the invention, anintervertebral implant includes an upper surface generally conforming toa plane and a lower surface generally conforming to a plane. A fastenerhole having a longitudinal axis extends transversely through one of theplane of the upper surface and the plane of the lower surface. Theimplant further includes means for seating a fastener in the fastenerhole and translating the seated fastener within the fastener hole.

In another exemplary embodiment of the invention, an intervertebralimplant includes an upper surface generally conforming to a plane, alower surface generally conforming to a plane, a slot having alongitudinal axis extending transversely through one of the plane of theupper surface and the plane of the lower surface, and an elongated seatat an end of the slot for translating a seated fastener head within theslot.

In another exemplary embodiment of the invention, an intervertebralimplant includes an upper surface for at least partial engagement withan end plate of a first vertebrae, the upper surface generallyconforming to a plane, and a lower surface for at least partialengagement with an end plate of a second vertebrae, the lower surfacegenerally conforming to a plane. A first fastener hole has alongitudinal axis extending transversely through the plane of the uppersurface, and an elongated cross section. A second fastener hole has alongitudinal axis extending transversely through the plane of the lowersurface, and an elongated cross section.

In another exemplary embodiment of the invention, an intervertebralimplant includes an upper surface and a lower surface, and a slidingmember displaceable in a plane passing through the upper surface and thelower surface. A fastener slot extends through the sliding member, theslot having a longitudinal axis extending transversely through one ofthe upper surface and the lower surface.

In another exemplary embodiment of the invention, an intervertebralimplant includes an upper surface for at least partial engagement withan end plate of a first vertebrae, and a lower surface for at leastpartial engagement with an end plate of a second vertebrae. The implantfurther includes an anterior surface extending between the upper surfaceand the lower surface, and a cover detachably connected over theanterior surface. The cover may include an exterior surface following aconvex curvature. In addition, or as an alternative, the cover may becoated with a bioactive surface. The anterior surface of the implant mayinclude a plurality of fastener holes and a plurality of screw fastenersextending through the fastener holes, with the cover forming a fixtureon the implant preventing displacement of the screws out of the fastenerholes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary and the following description will be more clearlyunderstood in conjunction with the drawing figures, of which:

FIG. 1 is a perspective view of an exemplary implant in accordance withone embodiment of the invention, schematically showing the implantpositioned between two vertebral bodies;

FIG. 2 is a top cross-sectional view of a component shown in FIG. 1;

FIG. 3 is a front view of a component shown in FIG. 1;

FIG. 4 is a perspective view of an exemplary implant in accordance withanother embodiment of the invention;

FIG. 5 is a perspective view of an exemplary component in accordancewith another embodiment of the invention;

FIG. 6 is a front view of the component of FIG. 5, schematically showingthe component implanted between two vertebral bodies in a first setting;

FIG. 7 is a front view of the component of FIG. 5, schematically showingthe component implanted between two vertebral bodies in a secondsetting;

FIG. 8 is a front view of an exemplary implant in accordance withanother embodiment of the invention;

FIG. 9 is a perspective view of an exemplary component in accordancewith another embodiment of the invention;

FIG. 10 is a front view of the component of FIG. 9, schematicallyshowing the component implanted between two vertebral bodies in a firstsetting;

FIG. 11 is a front view of the component of FIG. 9, schematicallyshowing the component implanted between two vertebral bodies in a secondsetting;

FIG. 12 is a perspective view of an exemplary implant in accordance withanother embodiment of the invention;

FIG. 13 is an exploded view of the implant of FIG. 12, with some of thecomponents separated from one another;

FIG. 14 is a front view of an exemplary implant in accordance withanother embodiment of the invention, showing the implant in a firstsetting;

FIG. 15 is a front view of the implant of FIG. 14, showing the implantin a second setting;

FIG. 16 is a schematic side view of the implant of FIG. 14 in the firstsetting;

FIG. 17 is an exploded view of components of the implant of FIG. 14;

FIG. 18 is a front view of an exemplary implant in accordance withanother embodiment of the invention;

FIG. 19 is a front view of an exemplary implant in accordance withanother embodiment of the invention;

FIG. 20 is a perspective view of an exemplary implant in accordance withanother embodiment of the invention;

FIG. 21 is an exploded perspective view of components of the implant ofFIG. 20;

FIG. 22 is another perspective view of the implant of FIG. 20;

FIG. 23 is a front view of an exemplary implant in accordance withanother embodiment of the invention; and

FIG. 24 is a perspective view of an exemplary implant in accordance withanother embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

The fusion interbody assembly of the present invention improves uponprior approaches by addressing, among other issues, the occurrence ofsubsidence and settling of the implant into the surrounding vertebrae.Applicants have discovered a number of problems and drawbacks associatedwith interbody cages that use rigidly constrained fixation members, suchas bone screws that are fixed in angulation and position relative to theinterbody. Interbody cages are capable of subsiding into endplates ofadjacent vertebra by as much as 6 mm. When fixation screws are rigidlyconstrained in the interbody, the fixation screws provide resistanceagainst subsidence, absorbing much of the load during settling. Thiscreates a number of concerns. First, the screws are not optimal forreceiving loads caused by settlement, and may be compromised if theseloads are excessive. Second, because the fixation screws absorb theloads that occur from subsidence, the fixation screws and interbodyprovide stress shielding to the bone graft material. Wolff's Lawrecognizes that bone material is a living structure that adapts to loadsand remodels itself over time to accommodate the loads. Bone materialgrows and becomes stronger in response to increased stresses. If bonegraft material is shielded from loads that occur during settlement, bonegrowth will be inhibited and fusion will not occur properly. Subsidencehas been linked to pseudoarthrodesis and non-union of the fusion site.

The various embodiments of the present invention allow proper loaddistribution to the bone graft material during subsidence, while stillutilizing fixation members like bone screws within the disc space. Thisis accomplished by allowing fixation to occur over time as a dynamicprocess in response to subsidence and settling. Rather than absorb loadsthat occur during subsidence, the bone screws are permitted to translateand/or pivot with respect to the interbody as the implant subsides. Thisallows the subsidence loads to be transferred to the bone graftmaterial, rather than be absorbed by the bone screws. To accomplishthis, the embodiments include translation mechanisms that allow thefixation members to translate and pivot in response to subsidence andsettlement, while maintaining the fixation members firmly anchored inthe implant. The translation mechanism may take a number of forms, aswill be appreciated from the exemplary embodiments described in thefollowing sections.

Referring now to FIGS. 1-3, an exemplary intervertebral implant 10 inaccordance with one embodiment of the invention is shown. Implant 10includes a generally ring-shaped body 20 inserted between two vertebraefor fusion of the vertebrae. Body 20 forms a hollow interior 22 thatprovides a bone graft space 24. A plurality of bone fixation members 90are used to secure implant 10 in the disc space. Bone fixation members90 cooperate with other components of implant 10 to provide adynamically controlled fixation that accommodates subsidence of theimplant while maintaining the rigidity of connection between thefixation members, the body and the vertebrae.

Implants in accordance with the present invention may be secured using anumber of different screw configurations, or other types of anchors.Because subsidence of the implant can cause some anchors to loosen or“back out” of their holes in the vertebrae, the implants of the presentinvention preferably include anchors having a locking mechanism thatlocks a portion of the anchor to the implant, while still permittingtranslation and pivoting during subsidence. For example, a bone screwwith a head that locks into a receiving part may be used, such as thatshown in International Pub. No. WO 2006/040063 A1 to Peukert, et al.,the contents of which is incorporated by reference herein. The Peukertscrew features a slotted head with a mechanism for expanding the headoutwardly. Expansion of the screw head forms a compression fit with thereceiving slot to protect against screw back out.

The implants in accordance with the present invention may be used invarious areas of the spine, including areas in the cervical, thoracicand lumbar regions of the spine. Implant 10 is schematically shownimplanted between two lumbar vertebrae, L3 and L4. This location isshown for illustrative purposes only, and implants in accordance withthe invention may be used between other vertebrae. An upper surface 30engages a superior end plate P1 on vertebra L3, and a lower surface 40engages an inferior end plate P2 on vertebra L4. For purposes of thisdescription, the term “upper” will refer to features that face or extendtoward a superior vertebra in the implanted state, the term “lower” willrefer to features that face or extend toward an inferior vertebra in theimplanted state, the term “anterior” will refer to features that face orextend toward the anterior side of the spine in the implanted state, andthe term “posterior” will refer to features that face or extend towardthe posterior side of the spine in the implanted state.

Upper and lower surfaces 30, 40 are generally planar. It will be notedthat the generally planar upper and lower surfaces may feature a smallcurvature, 25 such as a slight convex curvature as shown with surfaces630, 640 in FIG. 16, or be more or less flat. Regardless of the generalshape of the upper and lower surfaces, implants of the present inventionpreferably include textured or irregular surfaces to enhance ingrowth ofadjacent bone. In FIG. 1, for example, upper surface 30 includes anengagement surface 32 with a plurality of sharp ridges 34. Similarly,lower surface 30 40 includes an engagement surface 42 with a pluralityof sharp ridges 44. The ridges 34, 44 are configured to engage theirrespective end plates and provide a firm connection that resistsslippage or shifting of the implant in the anterior-posterior plane.

An anterior surface 50 extends between upper and lower surfaces 30, 40.A plurality of fastener holes 60 extend through body 20 on the anteriorside, and penetrate through anterior surface 50. Each fastener hole 60is adapted to receive an elongated fixation member that anchors the body20 in the intervertebral space. A number of fastener hole configurationsmay be used in accordance with the present invention, as will beappreciated from the embodiments to be described. Body 20 includes apair of inner fastener holes 60 a and a pair of outer fastener holes 60b. Inner and outer fastener holes 60 a, 60 b each have a longitudinalaxis (i.e. axis extending in the direction of penetration through body20) that is sloping or transverse relative to the upper and lowersurfaces 30, 40.

Each inner hole 60 a has a longitudinal axis that intersects the planeof upper surface 30. In this arrangement, inner holes 60 a extend towarda superior vertebra in the implanted state. Conversely, each outer hole60 b has a longitudinal axis that intersects the plane of lower surface40. In this arrangement, outer holes 60 b extend toward an inferiorvertebra in the implanted state. Fastener holes 60 are also angledlaterally with respect to the implant, as shown in FIG. 2. Two of theholes 60 extend toward a left side 26 of body 20, and two of the holesextend toward a right side 28 of body 20. Fastener holes 60 arepreferably oriented in a symmetrical arrangement with respect to atleast one axis of the implant. In FIG. 2, for example, fastener holes 60are symmetrically arranged with respect to anterior-posterior axis “X.”Inner fastener holes 60 a have a diverging axis of entry with respect toaxis X, and outer fastener holes 60 b have a diverging axis of entrywith respect to axis X. A symmetrical arrangement of the fastener holes,and consequently the fixation members, provides a more balancedresistance to stress forces, and a more uniform hold.

Referring again to FIG. 1, fastener holes 60 each contain a fixationscrew 90. Each screw 90 includes a rounded locking head 92 similar tothose described in International Pub. No. WO 2006/040063 A1 and athreaded shank 94. Screws 90 that are inserted through inner fastenerholes 60 a extend through the bone graft space 24 and the plane of uppersurface 30. Screws 90 that are inserted through inner fastener holes 60b extend through the bone graft space 24 and the plane of lower surface40. Because screws 90 are able to translate and pivot in response tosubsidence, loads applied to body 20 and screws 90 are distributed intothe graft space.

The anterior surface and/or fastener holes may be arranged so as tominimize the risk of adverse interaction with blood vessels and otherparts of the anatomy on the anterior side of the spine. Anterior surface50 has a generally concave curvature 52 that bows inwardly toward thecenter of body 20. This arrangement reduces the potential for screwheads contacting blood vessels outside the disc space on the anteriorside of the spine.

As discussed above, implant 10 includes a translation mechanism 80 thatallows screws 90 to adjust in response to subsidence and settling of theimplant. Translation is provided in particular by slotted openings 82that coincide with fastener holes 60, and the cross-sectionalconfigurations of the fastener holes. Each slotted opening 82 is formedwith straight sides 83 and rounded ends 85. Straight sides 83 extendgenerally perpendicularly to the planes of upper and lower surfaces 30,40, forming elongated openings with long dimensions extending more orless parallel to the length of the spine. The dimensions of slottedopenings 82 are larger than the maximum expanded diameters of screwheads 92, thus being adapted to receive screws 90 and allow screw heads92 to pass through the slotted openings. Each fastener hole 60 has awidened cross section 62 in proximity to its respective slotted opening82. The cross-section of fastener hole transitions from widened crosssection 62 to a reduced cross-section 64. The transition between widenedcross section 62 and reduced cross section 64 forms a rounded seat 70.The cross-sectional dimensions of widened section 62 are larger than thediameters of screw heads 92, while the cross-sectional dimensions ofreduced section 64 are smaller than the diameters of the screw heads.Both sections 62 and 64 have cross-sectional dimensions that are largerthan the maximum dimensions of screw shanks 94. In this arrangement,each fastener hole 60 allows insertion of screw shank 94 through theanterior wall of body 20, with seat 70 preventing screw head 92 frompassing completely into reduced section 65.

Seat 70 generally follows the shape of slotted opening 82, withelongated sides and rounded ends forming a track 72. Track 72 provides asurface on which screw heads 92 can be slidably displaced when screws 90are inserted in fastener holes 60. The elongated shape of each track 72allows a limited range of linear translation of a screw head relative tobody 20. The range of linear translation is represented in FIG. 3 byarrows “L”. Seats 70 also permit a limited range of pivoting of screws90 and screw heads 92 with respect to anterior surface 50. This can bevisualized from the schematic illustration in FIG. 1.

Translation and pivoting of screws 90 is also facilitated by the crosssectional shapes of fastener holes 60. Rather than having cylindricalbores, fastener holes 60 have elongated cross sectional shapes thatallow the shank to both translate and/or pivot with respect to theanterior surface of body 20. The dimensions of fastener holes preferablylimit the amount of translation and pivoting to ranges that correspondto an expected amount of subsidence and settling.

The dimensions of the implant of the present invention may vary,depending on factors including the particular region of the spine andother parameters. For example, an implant body to be used in the lumbarregion may be larger than an implanted body to be used in the cervicalregion. Implants having a total height of about 11 mm and fixationplacement within about 17.5 mm of the midline are suitable for purposesof the invention, although implants with larger or small sizes anddifferent placements may also be satisfactory.

Referring now to FIG. 4, an exemplary implant 110 in accordance withanother embodiment of the invention is shown. Implant 110 has aring-shaped body 120 that features a substantially flat anterior surface150. The flat anterior surface forms a wider anterior wall on body 120.The wider anterior wall forms a relatively deep recess within which aplurality of bone screws 190 are inserted into fastener holes 160. Thescrew heads 192 on bone screws 190 are recessed inside the anteriorsurface, and are thus positioned further within the perimeter of theinterbody as compared to implant 10.

Referring now to FIGS. 5-7, an exemplary implant 210 in accordance withanother embodiment of the invention is shown. Implant 210 includes agenerally ring-shaped body 220 that forms a hollow interior 222. Hollowinterior 222 provides a space 224 for bone graft material. Body 220includes an upper surface 230, a lower surface 240 and an anteriorsurface 250. A plurality of fastener holes 260 extend through body 220on the anterior side, and penetrate through anterior surface 250.Fastener holes 260 are configured to gradually increase the amount ofresistance to subsidence, while still maintaining the ability todynamically translate as greater loads are applied. A pair of diverginginner holes 260 a extend outwardly and downwardly from anterior surface250 toward the plane of lower surface 240. A pair of converging outerholes 260 b extend inwardly and upwardly from anterior surface 250toward the plane of upper surface 230. Each of the inner holes 260 a andouter holes 260 b aligns with an elongated slot opening 282.

Fastener holes 260 cooperate with bone screws 290 to form a dynamicfixation. Each fastener hole 260 contains a seat 270 for engaging ascrew head 292 on a bone screw 290. In addition, each fastener hole 260includes an elongated cross-section 266. The seat 270 and elongatedcross-section 266 provide a translation mechanism 280 that allowssubsidence loads to be distributed to the graft area in a controlledmanner. FIGS. 6 and 7 schematically illustrate the dynamic translationof bone screws 290 during subsidence in one possible scenario. It isnoted that for implant 210, and all implants illustrated herein, theillustrated screw translations are exemplary. The actual orientation ofthe screws after translation may appear in a number of arrangements,including but not limited to arrangements where some screws translatemore than others. FIG. 6 illustrates the relative position of the screwsbefore subsidence of the implant into the adjacent end plates. FIG. 7illustrates the relative position of the screws after subsidence of theimplant into the adjacent end plates is complete. Each screw head 292undergoes a unidirectional translation. Because the seats 270 areoriented at oblique angles with respect to the anterior face, the forcerequired to translate the screw heads 292 relative to the seatsgradually increases so that greater and greater resistance totranslation is applied during subsidence. The slanted orientations ofseats 270 also gradually increase the holding strength of the implant assubsidence occurs. A variety of angular orientations may be used withsatisfactory results. In implant 210, fastener holes 260 are oriented sothat screws enter the implanted body 220 at an angle of 40 degreesrelative to the cranial-caudal axis and 30 degrees relative to themedial-lateral axis.

Referring now to FIG. 8, an exemplary implant 310 in accordance withanother embodiment of the invention is shown. Implant 310 is similar insome respects to implant 210, but includes fastener holes 360 havingdifferent axes of entry than those of implant 210. Among otherdifferences, implant 310 includes outer fastener holes 360 b thatdiverge from one another outwardly, rather than converge inwardly.

Referring now to FIGS. 9-11, an exemplary implant 410 in accordance withanother embodiment of the invention is shown. Implant 410 features aninterbody 420 with a large anterior recess 421. Recess 421 houses atranslation mechanism 480 that includes an outer plate 454 and an innerplate 482. Outer plate 454 forms a pocket 456 in which inner plate 482is mounted. Inner plate 482 is slidable in pocket 456 relative to outerplate 454. Two sets of generally cylindrical fastener holes 460 extendthrough inner plate 482 and outer plate 454. A pair of inner fastenerholes 460 a extend through inner plate 482 with axes that divergeoutwardly. A pair of outer fastener holes 460 b extend through outerplate 454 and converge inwardly. Outer plate 454 may include engagementsurfaces that conform to engagement surfaces on interbody 420. Forexample, outer plate 454 may include a plurality of ridges that conformto ridges 434 on an upper surface 430 of interbody 420. Outer plate 454may also include a pair of circular notches or cutouts 455, as shown inFIG. 9, that permit a portion of the bone screws or other fixationcomponents to pass upwardly through part of the outer plate and intograft space 424.

Inner plate 482 and outer plate 454 are displaceable and operableindependently in response to subsidence of the implant in the superiorend plate and/or inferior end plate. In particular, inner plate 482 isslidable in pocket 456 so as to move relative to outer plate 454 andinterbody 420. Outer plate 454 is slidable in recess 421 so as to movewith respect to interbody 420. Pocket 456 and recess 421 can be arrangedto limit the amount of relative displacement of the inner and outerplates 482, 454. For example, pocket 456 may be configured so as toallow a displacement of inner plate 482 by as much as 3 mm duringsubsidence. Recess 421 may be configured so as to allow a displacementof outer plate 454 by up to 1.5 mm. More or less displacement may bepermitted, depending on the anticipated amount of settling, thedimensions of components or other parameters. Outer plate 454 includes apair of engagement holes 452 that cooperatively engage an instrument,such as a guiding device, holder, retractor system or other tool formanipulating implant 410.

FIGS. 10 and 11 schematically illustrate the dynamic translation of aplurality of bone screws 490 during subsidence of implant 410 in onepossible scenario. In particular, FIG. 10 illustrates the relativeposition of bone screws 490 before subsidence of implant 410 into theadjacent end plates. FIG. 11 illustrates the relative position of bonescrews 490 after subsidence of implant 410 into the adjacent end plates.Prior to subsidence, inner plate 482 is positioned toward an uppersurface 430 of implant 410. After subsidence, inner plate 482 ispositioned toward a lower surface 440 of implant 410.

In some circumstances, it may be desirable to provide one or moremechanisms for additional stability after subsidence. Referring now toFIGS. 12 and 13, an exemplary implant 510 in accordance with anotherembodiment of the invention is shown. Implant 510 is similar to implant410 discussed above, but includes a fixation extension 583 for increasedstability. A pocket 556 opens toward an upper surface 530 of theimplant, and contains an inner plate 582. Inner plate 582 isdisplaceable in pocket 556 to permit translation of bone screws duringsubsidence of implant 510. FIG. 12 shows inner plate 582 in a positioncorresponding to how it would appear after implant 510 undergoessubsidence in one possible scenario. Inner plate 582 is displaced towarda lower surface 540 of implant 510. Fixation extension 583 projectsthrough the bottom of outer plate 554 and below lower surface 540. Thebottom portion of fixation extension 583 includes a sharp edge 584.Sharp edge 584 is configured to penetrate into the inferior end plateduring subsidence, providing additional stability of implant 510 andresistance to movement between the vertebrae.

FIG. 13 shows components of implant 510 separated from one another. Body520 includes a pair of cutouts 555 that permit outer bone screws 560b topass upwardly through the outer plate and into a graft space in theinterior of body 520.

Sliding members, such as the inner plates 482, 582 shown in FIGS. 9-13,may be mounted directly into an interbody, rather than a separate outerplate mounted in the body. FIGS. 14-17 show another exemplary implant610 in accordance with the invention that features direct mounting, andtherefore fewer components. Implant 610 has an anterior wall 650 with apocket 656 that contains an inner plate 682. Inner plate 682 is slidablydisplaceable within pocket 656 to allow dynamic fixation similar to theimplants discussed above. Inner plate 682 includes a pair of innerfastener holes 660 a with diverging axes of entry. Anterior wall 650includes a pair of outer fastener holes 660 b also having diverging axesof entry. FIGS. 14 and 15 schematically illustrate the dynamictranslation of bone screws 690 during subsidence of implant 610 in onepossible scenario. In particular, FIG. 14 illustrates the relativeposition of bone screws 690 before subsidence of implant 610 into theadjacent end plates. FIG. 15 illustrates the relative position of bonescrews 690 after subsidence of implant 610 into the adjacent end plates.

Inner plates may be secured into outer plates or interbodies using anumber of connection types. For example, implant 610 uses a sliding pin686 as seen best in FIGS. 16 and 17. Inner plate 682 includes a centralbore 687 that extends through the midsection of the inner plate. Slidingpin 686 extends through bore 687 and supports inner plate 682 at amidsection on the pin. The ends of pin 686 are slidably retained inslots 623 on left and right sides of interbody 620. Slots 623 areelongated in a direction generally perpendicular to the planes of theupper and lower surfaces of body 620. In this arrangement, slots 623permit a controlled range of movement of inner plate 682 relative tointerbody 620 in response to subsidence of implant 610.

The anterior ring apophysis of a vertebral body provides a relativelythick area of cortical bone forming a strong point of fixation for bonescrews. Therefore, it may be desirable to adjust the approach angles ofbone screws so that the shanks are oriented more directly toward theanterior ring apophysis region. FIGS. 18 and 19 illustrate exemplaryimplants in accordance with the invention that orient the bone screws soas to penetrate the anterior ring apophysis. FIG. 18 illustrates animplant 710 that includes a body 720 and translation means 780 thatincludes a fixed ring apophysis plate 782. Ring apophysis plate 782 hasa plurality of fastener holes 760 and elongated seats 770 within thefastener holes. Fastener holes 760 have elongated cross sections thatallow for translation of bone screws within the holes during subsidenceof implant 710. Ring apophysis plate 782 has an upper plate section 784containing a pair of inner fastener holes 760 a, and a lower platesection 786 containing a pair of outer fastener holes 760 b. Upper andlower plate sections 784, 786 extend outwardly from interbody 720 in ananterior direction, and are angled so as to direct the screw shanks intothe ring apophysis regions of the superior and inferior vertebrae.

FIG. 19 illustrates an implant 810 that includes a body 820 andtranslation means 880 that includes a sliding ring apophysis plate 882.Ring apophysis plate 882 has a plurality of fastener holes 860 that aregenerally cylindrical. Translation of bone screws is facilitated by apair of sliding plate members that form ring apophysis plate 882. Alower plate member 884 is fixedly mounted to body 820 and contains apair of outer fastener holes 860. An upper plate member 886 is slidablewith respect to the lower plate member 884 through a slot 885 in thelower plate member. The slot 885 in lower plate member 884 is adapted toreceive upper plate member 886 during subsidence of implant 810. Upperplate member 886 contains a pair of inner fastener holes 860 a, andlower plate member 884 contains a pair of outer fastener holes 860 b.Upper and lower plate members 886, 884 extend outwardly from interbody820 in an anterior direction, and are angled so as to direct the screwshanks into the anterior ring apophysis regions of the superior andinferior vertebrae.

Referring to FIGS. 20-22, an exemplary implant 910 in accordance withanother embodiment of the invention is shown. Implant 910 features aninterbody 920 and a detachable translation mechanism 980. Translationmechanism 980 includes a dynamic fixation plate 982 with slottedopenings 984. A plurality of bone screws 990 extend through slottedopenings 984. Fixation plate 982 includes a rigid body 983 thatreinforces the fusion site, while allowing for controlled translation ofbone screws during subsidence. The anterior face of fixation plate 982is substantially smooth with rounded edges to minimize damage to bloodvessels and other neighboring structures. The rear or posterior-facingside of fixation plate 982 includes a pair of tab connectors 988. Eachtab connector 988 has a pair of opposing catch tabs 988 a thatdetachably engage an anterior side 950 of interbody 920. Anterior side950 includes a pair of slots 951 adapted to receive tab connectors 988.Catch tabs 988 a are configured to flex inwardly during entry into andpassage through slots 951, and then snap outwardly after exiting theslots into the hollow interior 922 of body 920. Catch tabs 988 a restagainst the inner wall surrounding the hollow interior 922 to preventreversing the tab connectors 988 out of slots 951, as shown in FIG. 22.

Referring now to FIG. 23, an exemplary implant 1010 in accordance withanother embodiment of the invention is shown, which features aninterbody 1020 and a translation mechanism in the form of a dynamicbuttress plate 1082. Buttress plate 1082 is slidably received in ananterior slot 1056 on interbody 1020 to adjust for subsidence of theimplant 1010. A pair of screws 1090 are inserted through fastener holes1060 as shown to attach buttress plate 1082 to a vertebral body. Oncebuttress plate 1082 is secured to the vertebral body, the buttress plateslides in the direction shown by the arrows.

As noted above, the anterior surface of the implant may lie in proximityto blood vessels that can be injured by sharp edges or projections onthe implant. Where this is a risk, embodiments of the invention mayinclude smooth contours on their anterior surfaces, such as roundedcorners and recessed fastener holes, to minimize the potential forinjury to surrounding blood vessels, soft tissue and other structuresduring subsidence and movement of the implant and its components. Smoothcontours may be used on the anterior surface of the interbody, or on aseparate component placed over the anterior surface of the interbody.For example, the anterior surface of the interbody may be partiallyenclosed inside a cover or shield.

Referring to FIG. 24, an exemplary cover 1158 is shown on an interbody1120. Cover 1158 extends over an anterior surface 1150 of interbody 1120to enclose the heads of fasteners 1190 inserted into the interbody. Asmooth rounded exterior surface 1159 on cover 1158 forms a continuousrounded perimeter with the sidewalls of interbody 1120. Rounded surface1159 may contact and slide against blood vessels, tissue and othersensitive structures without damaging them. Cover 1158 extends overfasteners 1190 without contacting or interfering with the fastener headsin the slotted openings 1160. Interbody 1120 and cover 1158 may includecooperating tabs and slots, or other types of connectors to facilitateattachment of the two components.

A cover such as cover 1158 provides several advantages. For example, asnoted above, a cover can provide a relatively smooth surface thatcontacts and slides against blood vessels without damaging them. As arelated benefit, a cover provides a protective barrier thatsubstantially prevents back out of screws into blood vessels or othernearby structures. A cover further provides additional mass on theanterior portion of interbody 1120, increasing the strength of theanterior load column of the vertebral body. Moreover, a cover canprovide an active surface that promotes attachment to adjacent tissuecells. The cover may include a bioactive coating, for example, thatpromotes or controls adhesion of tissue cells. Tissue cells that attachto the cover may further stabilize the implant and provide a source forsecondary fixation that is not influenced by subsidence or relativemovement of the fasteners in the interbody, which are separated from thetissue by the cover.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. For example, the interbodies, platesand other components that are illustrated herein with round fastenerholes may alternatively feature slotted fastener holes to alter thedynamic characteristics of those components and their respectiveimplants. Accordingly, it is intended that the appended claims cover allsuch variations as fall within the scope of the invention.

1. An intervertebral implant comprising: an upper surface generallyconforming to a plane; a lower surface generally conforming to a plane;a fastener hole having a longitudinal axis extending transverselythrough one of the plane of the upper surface and the plane of the lowersurface; and means for seating a fastener in the fastener hole andtranslating the seated fastener within the fastener hole.
 2. Theintervertebral implant of claim 1 wherein the means for seating afastener in the fastener hole and translating the seated fastener withinthe fastener hole comprises an elongated seat surface at one end of thefastener hole and an elongated cross section within the hole, theelongated cross section extending along the entire longitudinal axis ofthe hole.
 3. The intervertebral implant of claim 2 comprising anelongated fixation member having a head engaging the seat surface, theseat surface forming a track in which the head is limited to linearmovement with respect to the seat surface.
 4. The intervertebral implantof claim 1 wherein the means for seating a fastener in the fastener holeand translating the seated fastener within the fastener hole comprises asliding member displaceable with respect to the upper and lowersurfaces, the fastener hole passing through the sliding member.
 5. Theintervertebral implant of claim 1 wherein the fastener hole comprises afirst fastener hole having a longitudinal axis extending transverselythrough the plane of the upper surface, the implant further comprising asecond fastener hole having a longitudinal axis extending transverselythrough the plane of the lower surface.
 6. The intervertebral implant ofclaim 5 further comprising a third fastener hole having a longitudinalaxis extending transversely through the plane of the upper surface, anda fourth fastener hole having a longitudinal axis extending transverselythrough the plane of the lower surface.
 7. The intervertebral implant ofclaim 6, wherein the longitudinal axis of the first fastener hole isgenerally parallel to the longitudinal axis of the third fastener hole,and the longitudinal axis of the second fastener hole is generallyparallel to the longitudinal axis of the fourth fastener hole.
 8. Anintervertebral implant comprising: an upper surface generally conformingto a plane; a lower surface generally conforming to a plane; a slothaving a longitudinal axis extending transversely through one of theplane of the upper surface and the plane of the lower surface; and anelongated seat at an end of the slot for translating a seated fastenerhead within the slot.
 9. The intervertebral implant of claim 8, whereinthe implant comprises an anterior face, and the slot extends through theanterior face, with the seat recessed in the anterior face.
 10. Theintervertebral implant of claim 9 further comprising an anterior plate,the anterior face being formed on the anterior plate.
 11. Theintervertebral implant of claim 10, wherein the anterior plate comprisesa flexible slotted plate extending in a plane generally normal to theplanes of the upper and lower surfaces.
 12. The intervertebral implantof claim 9, further comprising a cover plate detachable connected withthe anterior face of the implant.
 13. An intervertebral implantcomprising: an upper surface for at least partial engagement with an endplate of a first vertebrae, the upper surface generally conforming to aplane; a lower surface for at least partial engagement with an end plateof a second vertebrae, the lower surface generally conforming to aplane; a first fastener hole having a longitudinal axis extendingtransversely through the plane of the upper surface, and having anelongated cross section; and a second fastener hole having alongitudinal axis extending transversely through the plane of the lowersurface, and having an elongated cross section.
 14. The intervertebralimplant of claim 13, wherein the implant comprises an anterior face, thefirst and second fastener holes extending through the anterior face andforming elongated slotted seats in the anterior face.
 15. Theintervertebral implant of claim 14 further comprising an anterior plate,the anterior face being formed on the anterior plate.
 16. Theintervertebral implant of claim 15, wherein the anterior plate comprisesa flexible slotted plate extending in a plane generally normal to theplanes of the upper and lower surfaces.
 17. The intervertebral implantof claim 14, further comprising a cover plate detachably connected withthe anterior face of the implant.
 18. An intervertebral implantcomprising: an upper surface and a lower surface; a sliding memberdisplaceable in a plane passing through the upper surface and the lowersurface; and a fastener slot extending through the sliding member, theslot having a longitudinal axis extending transversely through one ofthe upper surface and the lower surface.
 19. The intervertebral implantof claim 18 comprising an anterior plate forming a pocket, the slidingmember being slidably displaceable in the pocket.
 20. The intervertebralimplant of claim 18, comprising a fixation extension extending from thesliding member, the extension being displaceable in unison with thesliding member to an extended position, in which the extension projectsthrough a plane passing through one of the upper and lower surfaces. 21.The intervertebral implant of claim 18, comprising a ring-shaped bodywith a hollow interior, the hollow interior forming a bone graft space.22. The intervertebral implant of claim 21 wherein the fastener slotcomprises a first slot passing through the sliding member and having alongitudinal axis extending transversely through the bone graft space.23. The intervertebral implant of claim 21, wherein the fastener slotcomprises a first slot, the implant further comprising a second slot,the first and second slots both passing through the sliding member andhaving a longitudinal axis extending transversely through the bone graftspace.
 24. An intervertebral implant comprising: an upper surface for atleast partial engagement with an end plate of a first vertebrae; a lowersurface for at least partial engagement with an end plate of a secondvertebrae; an anterior surface extending between the upper surface andthe lower surface; and a cover detachably connected over the anteriorsurface.
 25. The intervertebral implant of claim 24, wherein the covercomprises a exterior surface following a convex curvature.
 26. Theintervertebral implant of claim 24, wherein the cover is coated with abioactive surface.
 27. The intervertebral implant of claim 24, whereinthe anterior surface comprises a plurality of fastener holes and aplurality of screw fasteners extending through the fastener holes, thecover forming a fixture on the implant preventing displacement of thescrews out of the fastener holes.